Can you log in to the database with SQLPlus as USER_NAME/password@saturnsring and run
SELECT ALRT_SEQ.nextval FROM DUAL
from the sql prompt?
To me this looks like a database set-up problem, not a jboss or XADatasource problem. Do you have a public synonym for the system table dual('system.dual') call 'dual'? Conversely has USER_NAME been granted select on dual or system.dual?
Check these out first and then let me know. It sure looks like the USER_NAME schema can't see the DUAL table (or public synonym)